The field of the present invention relates generally to geophysical exploration utilizing seismic techniques. More particularly, this invention provides a method and apparatus for adaptive processing of seismic signals during the course of acquisition of seismic data.
Seismic exploration involves imparting seismic wave energy into the earth's formations with a seismic source. Typical of such seismic sources are explosive charges and mechanical seismic wave generators. The seismic wave energy is reflected and/or refracted due to differences in acoustic impedance of adjacent subsurface formations. Seismic wave energy detectors, i.e. seismometers, spaced about the surface of the earth develop electric analog signals in response to the reflected and/or refracted seismic wave energy.
The analog signals are collected at seismic data acquisition units such as described in Broding, et al., U.S. Pat. No. 3,806,864. The Broding seismic data acquisition units amplify, filter and digitize the analog signals such that they can be processed by a digital computing unit to produce a seismic trace. The seismic trace is a geophysical representation of the processed digital signals which permits seismologists to interpret and analyze the earth's subterranean formations.
Heretofore, seismic data acquisition units filtered the analog signal to eliminate selected frequencies which were not developed in the seismic process, e.g., 60 Hz noise from electric transmission lines, as well as to preclude the generation of alias frequencies in the subsequent digitization process. The generation of alias frequencies in the digitization process is the result of sampling the analog signal at a rate less than two samples per cycle of the highest frequency in the analog signal. J. A. Coffeen in "Seismic Exploration Fundamentals," Penn-Well Publishing Co (1978), page 87, notes that alias frequencies can be precluded by utilizing an anti-alias filter which is impressed on the analog signal to eliminate high frequencies which can generate alias frequencies depending upon the sampling rate utilized in the analog to digital conversion.
It is oftentimes desirable to resample the digitized analog signal, i.e., the digital signal. Presently, varying resampling rate of the digital signal occurs either after the acquisition period on a central processing unit or before the acquisition period by modifying the data acquisition unit. Resampling the digital signal, i.e., increasing the sample interval, has the effect of decreasing the volume of seismic data which are acquired. As such, resampling can be utilized as a method for effectively prolonging the operational period of utility for a portable seismic data acquisition unit before its seismic data storage capacity is exceeded. However, resampling the digital signal requires an anti-alias filter, adaptive to each resampling rate, be provided to preclude the generation of alias frequencies. Heretofore, a significant limitation in providing multiple resampling rates in a portable seismic data acquisition unit has been the inability to cooperatively couple the resample rate and the frequency domain of the anti-alias filter such that the resampling rate can be varied during the course of acquiring the seismic data without nonlinear phase response thereof.
Portable seismic data acquisition units, such as disclosed in Broding, et al., have been limited to the utilization of analog filters. The analog filters are high order Butterworth or high order Cauer type. Such filters suffer from both fundamental and practical limitations which compromise their usefulness for adaptive low pass frequency filtrating. Fundamentally, such analog filters have a nonlinear phase response characteristic in the passband which is evidenced as a differential time delay for frequency components of the analog signal. Conversely, sufficiently sharp frequency cutoff characteristics are not available with linear phase response analog filters of any practical order. Practically, component tolerances and nonideal behavior in realizable components make it difficult to construct an analog filter which has unit-to-unit repeatability because temperature effects cause the amplitude and phase characteristics of such analog filters to drift, resulting in a change in filter characteristics with temperature; fine adjustment of frequency characteristics is made difficult by complex interaction between components; high order analog filters have high component counts requiring substantial circuit board area; precision frequency characteristics require components which have close tolerances, high cost and may require hand selection; and relatively high power consumption which can be a considerable limitation in battery operated, portable seismic data acquisition units.
While most analog filters are limited to fixed passband frequency domains, this fact poses no limitation on the use of analog filters strictly as an anti-alias filter in conjunction with an analog to digital converter having a fixed sampling rate. However, if an anti-alias filter having bandpass frequency domains cooperatively coupled to a plurality of resampling rates is desired, the resulting analog filter design is very cumbersome and further exacerbates the problems of design fixed passband frequency domain analog filters previously noted.
Various digitally controlled analog filters have been proposed to provide multiple bandpass frequency domains for portable data acquisition units. A first approach has been to build a plurality of physically distinct analog filters, each with a specific bandpass frequency domain and select the appropriate analog filter with a solid state switch activated by a digital code. The disadvantages of this approach are the amount of power and circuit board area required, as well as the difficulty in selecting components and tuning the individual analog filters.
A second approach is to provide multiple integrated circuit switches for switching a multiplicity of passive devices in a basic analog design. In such a circuit, all the necessary capacitors or resistors and operational amplifiers would be fixed, and independent sets of resistors or capacitors corresponding to different frequency characteristics could be present on the board. A digital code would then select one of the available resistor or capacitor network sets to provide the desired bandpass frequency domain. Although superior to using multiple independent analog filters, there are still significant manufacturing problems necessitated by the care in selecting components and tuning the analog filter. A significant disadvantage in any realizable semiconductor circuit switch is its channel impedance which is variable with temperature. As such, temperature-related changes upon the resistance of the switching element upsets the critical relationship between component values which determine the frequency response of the analog filter.
A third approach is essentially the same as the second with the exception that miniature latching relays are used in place of the integrated circuit switching elements to provide a frequency selection. While this approach reduces switch impedance to negligible levels, all of the problems of component selection and analog filter tuning remain. Reliability is decreased since the relay mechanisms are relatively delicate and the shock and vibration requirements for its intended use with portable data acquisition units contraindicate the use of electromechanical devices of any sort.
Moreover, with any analog-type filter, it is virtually impossible to change the bandpass frequency domain during the acquisition of data without using a separate filter for each desired frequency domain. If multiple filters are used, all filters would have to be active at all times to respond immediately to a change in filter selection. If a single switched-component filter configuration were implemented, it would not be possible to change the frequency response in the single analog filter during acquisition without sacrificing several data samples because the bandwidth is finite and the slew rate is limited. Therefore a sudden change in the frequency domain would have a finite settling time distorting several samples following such a change. As such, providing portable data acquisition units with a multiplicity of selectable resampling rates without the loss or distortion of the data when changing resampling rates has not been practicable.